It is known that a chemical coagulant, such as alum, may be added to a bioreactor to precipitate soluble phosphates to reduce phosphorous discharge. It is also known that biological phosphorus removal is augmented by addition of metal salts such as ferric chloride or alum. These can be added directly into the aerobic zone of a reactor to chemically bind the phosphate. It is also known that alum coagulant enhances the filterability of a mixed liquor sample.
Submerged membrane bioreactors (MBR) generally have a limited capacity to handle hydraulic peak loads. As such, new wastewater treatment plants are substantially overdesigned to handle typical average annual flows or even increased flows during maximum monthly flows. Designing a MBR plant to handle the highest expected peak instantaneous flow often drives up capital costs and increases operating costs.
Energy costs can make up more than 50% of the total operating cost of a wastewater treatment plant using low pressure membrane filtration. In MBR plants, keeping hydraulic capacity online to handle a transient instantaneous flow means sustaining a large inventory of biosolids. The upkeep of biosolids during periods of low flow is expensive, as the microorganisms require oxygen to live. Oxygen is supplied through diffused air driven by blowers. Blower operation for membrane air scouring of membranes and oxygen delivery can make up 25% to 50% of the total electrical load in a MBR plant depending on the design.
The peak hydraulic capacity of submerged membrane bioreactor systems is generally on the order of twice the rated average capacity of the facility. In certain regions where heavy rainfall and or rapid snow melt are prevalent, short-term hydraulic peaks can vary between 2-10 times average flows for hours, days and even weeks. For systems with such large transient peaking factors, it is cost prohibitive to build and operate a MBR.
For wastewater treatment facilities designed for biological nutrient removal (nitrogen and phosphorous), one of the challenges is to deal with additional load coming from sludge digestion step, in the form of a supernatant. During aerobic or anaerobic digestion of the waste solids, the phosphorous and ammonia rich supernatant stream is generated which needs to be treated prior to discharge. In a prior art system, influent enters a process at an anoxic zone for denitrification. Denitrified wastewater is sent to an aerobic zone for nitrification and removal of soluble biochemical oxygen demand. Partially biologically treated wastewater is then fully nitrified and filtered in a membrane zone. Permeate can be subjected to additional treatment depending on the application (e.g. uv disinfection).
In this typical process, it is generally cost prohibitive to construct dedicated equalization to pick up more than very short flows lasting on the order of hours that exceed the filtration capacity of the membranes. Even capturing peak hour flows can escalate costs depending on the hydraulic peak and the duration of the event.
Also, in a prior art system, the supernatant from the digestion step is sent back to the submerged membrane bioreactor system.